Before electronic thermostats were available, electromechanical thermostats relied on energy provided by the temperature changes in the controlled space to operate a very low force switch, typically a mercury switch, and hence had no need for electrical operating power. These thermostats typically activated the heating or cooling plants they controlled by switching low voltage (i.e., 24 v.) AC power to a HVAC controller. The HVAC controller then switched 24 v. AC power to the various components of the plant, such as the gas valve and igniter in the heating plant or to the contactor controlling high voltage power for the air conditioning compressor and fan. But the electronic thermostats widely used nowadays typically require a constant supply of electrical power. In typical existing installations, two or three wires connect the thermostat to the plant. To run additional wires solely for operating power is often quite labor intensive, and hence relatively expensive. If external power is to be used, additional wires are necessary because when the thermostat switch closes, no power is available across the switch at the thermostat from the wires supplying power to the heating or cooling plant while the plant is operating. The voltage drop across the thermostat""s internal switch element is essentially zero while the switch is closed. Adding impedance to the switch to create a constant small voltage drop is undesirable because this generates heat within the enclosure, affecting thermostat operation. And for some controllers, impedance in the wires supplying power may affect controller operation.
Alternative internal power sources for thermostats are problematic. As previously mentioned, running a separate set of power wires to the thermostat solves the problem of supplying power for thermostats, and in new construction this is easy to do. But for older buildings in which an electronic thermostat is being retrofitted, this may be very difficult.
To address these problems, I earlier developed an alternative power source for thermostats that involved what I call power stealing. This power source is an electronic circuit that, while the thermostat is conducting current to the controller, takes a small section (typically no more than a few tens of xcexcsec.) of the beginning of each half of the low voltage AC cycles of the controller current, converts it to DC, and filters and regulates the stolen power voltage to the 3 v. that the thermostat circuitry uses. The amount of power stolen is so small that the HVAC controller operation is unaffected. I implemented this power-stealing function in an electronic circuit that uses little power and takes little space on the circuit board of the thermostat. My U.S. patents numbered U.S. Pat. No. 5,768,116 (issued Jun. 16, 1998) and U.S. Pat. No. 5,903,139 (issued-May 11, 1999) describe the power-stealing circuit and its operation, and these patents are incorporated by reference into this specification.
Briefly, the power-stealing circuit controls a pair of power field effect transistors (power FETs) to switch 24 v. AC to the HVAC controller or other load. When the controller is receiving power, the power-stealing circuit turns the power FETs off for a short period of time before and after each time the voltage crosses the 0 v. line and when the power voltage is relatively low. This allows the circuit to resynchronize with the AC wave, and to divert load current to a 3 v. supply capacitor. When the power voltage reaches 4 v. or so in each half cycle and the capacitor is fully charged, the power-stealing circuit causes the power FETs to again conduct and redirect power away from the 3 v. capacitor. The power xe2x80x9cstolenxe2x80x9d from each half cycle immediately after each zero crossing and used to charge the 3 v. capacitor then provides the operating voltage for the thermostat control circuitry.
Some thermostats and other similar types of controllers operate on line voltage, typically either 117 or 220 v. AC. The current version of the power-stealing circuit is implemented in an ASIC (application-specific integrated circuit) that is not compatible with such voltage levels. It would be possible to redesign the ASIC for compatibility with line voltage. However, this is not presently preferred because of the expense of redesigning and requalifying the existing 24 v. version of the ASIC.
I have developed an interface circuit that allows an existing power-stealing control circuit designed for low AC voltage power to operate successfully with line voltage power. Such an interface circuit cooperates with a first semiconductor switch of the type having a control terminal and first and second power terminals and similar to the power FETs used for switching power by the original design. The first semiconductor switch has first and second power terminals for connection in series with the load to control power applied to the load. The first switch conducts between the first and second power terminals responsive to a conduction signal at the control terminal. The control circuit has first and second power terminals connected in parallel with the first semiconductor switch power terminals. The control circuit has a switch terminal for receiving an on/off signal having first and second levels, and a switch control terminal providing the conduction signal to the first semiconductor switch for a period of time between successive zero crossings by the AC voltage responsive to the first level of the on/off signal. A storage element cooperates with the control circuit to store power available across the first semiconductor switch while the conduction signal is absent, and provides the stored power as a low DC voltage.
The interface circuit allows the existing low voltage control circuit design to operate with line AC voltage levels. The interface circuit comprises a variable impedance element connected between the first semiconductor switch first power terminal and the control circuit first power terminal. The variable impedance element functions to clip the voltage at the control circuit first power terminal to a peak voltage on the order of the low voltage AC power.
In a preferred embodiment the interface circuit includes a first variable semiconductor impedance having a first power terminal connected to the first semiconductor switch first power terminal, a second power terminal connected to the control circuit first power terminal, and a control terminal. The first variable semiconductor impedance has an impedance value dependent on the voltage between the control terminal and the second power terminal of the first variable semiconductor impedance. A voltage reference element provides a voltage to the first variable semiconductor impedance on the order of the peak voltage of the relatively low AC voltage. When the load is not receiving power, the first variable impedance assumes an internal impedance that creates sufficient voltage drop to hold the voltage applied to the control circuit to a level on the order of the low AC voltage level.